Many "smart" battery units include a battery and a device that gives information about the battery to such devices as battery chargers and other host devices that are powered by the battery (e.g., radios, telephones, etc.). One such device is a code resistor that identifies characteristics of the battery. The value of a code resistor may be determined by the battery charger, based on a current supplied to bias the code resistor and the voltage sensed across the code resistor. A smart battery unit might also include a battery monitor circuit that provides information to the host device during use.
Existing smart battery units allow the code resistor to be connected to the host device when the battery unit is also connected to a battery charger. Current to the code resistor could be supplied by both the battery charger and the host device. This results in a voltage across the code resistor being sensed by the battery charger that is different from the expected voltage for a given battery type. This can cause the battery charger to go off-line or to attempt to charge the battery in an incorrect manner.
Therefore, there is a need for a circuit that isolates a code resistor in a battery unit from a load when the battery unit is also connected to a battery charger.